It was supposed to be peak relaxation. Kate Johnson and her husband, Sean, planned a 10-day ski vacation to Japan early last year. They figured it would be their last big trip for a while since they were trying to get pregnant. A few weeks before boarding the plane, the Hoboken couple found out they were expecting. “I weighed my options on being able to ski and figured that so early on in the pregnancy, it wouldn’t have that much of an effect,” Johnson, 38, told The Post. “Little did I know how